1. Technical Field of the Invention
The present invention relates to a method and apparatus for measuring the alignment of an element.
2. Prior Art
There are many instances, particularly in the construction industry, where it is necessary to determine the so-called verticality of an element. Often the elements are not 100% straight, for example steel sections are supplied to certain rolling tolerances, and may therefore be slightly bowed. Therefore, it is not strictly accurate to refer to the verticality of an element; it is more appropriate to refer to the alignment of two (or more) levels of the element. Within the trade, the use of the word “vertical” is often interpreted as meaning that the location of two plan positions (i.e. the 2-D cross-section of the element at two different levels), are in vertical alignment.
Examples of instances where there exists a requirement to measure the alignment include the following:
1) Monitoring the alignment or “verticality” of a mast during continuous flight auger (CFA) piling techniques, or of a leader (typically a longitudinal frame fitted to the jibs of crawler base machines) during driven piling operations;
2) Monitoring the alignment of stop-ends placed in diaphragm wall panels both in and out of the plane of the diaphragm wall;
3) To monitor the verticality of pile bores and diaphragm wall panels;
4) During driven techniques to monitor the installation of the tube of a cast-in-situ pile, a pre-cast pile section, a steel section or sheet pile etc;
5) To ensure that a lift ram liner or bore for lift ram is plumb;
6) In continuous flight auger (CFA) piling operations, the verticality of the auger should be measured during the setting up of the rig and also once the auger has reached the founding depth;
7) To monitor casings which may be vibrated or driven to depth.
8) During the placement of columns to structural steel tolerances, within pile bores and diaphragm wall panels.
Although in the majority of instances there is a requirement for the two (or more) plan positions of the element to be in vertical alignment (i.e. the “x” and “y” co-ordinates of the element cross-sections at two levels are the same), there are also occasions where it is necessary to ensure that an element is positioned at a predetermined angle, so that a plan position at or near the top of the element is at an angle (in either/both the x- or y-direction) with respect to the plan position at or near the bottom of the element. For example, elements which provide foundation support for bridges are normally installed at a predetermined angle. It should therefore be noted that references to alignment or desired alignment should not be interpreted as necessarily being in vertical alignment, but may also be interpreted as encompassing a deliberate mis-alignment in either or both the x or y direction.
A number of techniques have been developed which allow the plan position of the lower level of an element to be brought into the desired alignment with the plan position of the upper level. Known surveying techniques suffer from the disadvantage that the equipment employed (for example, a theodolite) requires skilled technicians. Furthermore, it is necessary to locate the equipment at a significant distance from the element so that the base and head of the element can be viewed.
Another method involves the use of a simple plumb-bob or, alternatively, a laser plumb device. This comprises a laser emitting means fitted at or near the upper level and a target positioned at or near the lower level. The centre-line of the laser and the target are usually located at a fixed offset from the true centre-line of the element. A remote camera or binoculars may be needed to view the laser spot on the target. However, laser plumb bobs require regular calibration to ensure accuracy, a process which is time-consuming and involves the use of a frame of substantial length.
All of the above methods are of limited use when only one end of the element is visible. Moreover, none of the methods above can be applied when the element is hidden, for example, if the element is lowered down a hole filled with fluid.